It is well known to apply an electroless metal plating to an insulating substrate, such as plastic, by depositing a noble metal such as palladium, gold or platinum as an initiator or catalyst on the surface of the insulating substrate and then dipping the substrate into an electroless metal plating solution. It is also known to catalyze a surface with a non-noble metal catalytic layer, e.g., Cu, by reduction of a copper compound adsorbed on the surface.
Various techniques have been proposed to form an electric circuit by electroless metal plating on flexible or rigid substrates for the manufacture of printed circuits. Among the methods utilized to form the printed circuit boards is the method disclosed in U.S. Pat. No. 3,929,483 wherein a photoresist is laid on a surface which is highly absorbent to silver salts. Such a surface is anodized aluminum. Photoresist deposited on the surface is then exposed to light through a negative to attain a desired circuit pattern. The portions not struck by light are removed by washing in a suitable solvent. Thereafter, the substrate is soaked in a concentrated solution of silver nitrate wherein the silver salt is held by the adsorptive surface not coated by the resist. This silver salt is then reduced to metallic silver by treatment with a suitable reducing agent. The photoresist is then removed by treatment with an appropriate solvent leaving the desired silver image on the adsorptive media. The silver metal deposit acts as a catalyst for subsequent electroless plating.
In another electroless deposition method for the formation of a printed circuit board, the substrate is first sensitized and activated to form a blanket catalyzed layer thereon. Thereafter the resist pattern is formed over the catalyed layer and the substrate is then treated with an electroless metal deposition solution. In this way, metal deposits only on the sensitized, activated, exposed substrate area and not on the resist covered areas. However, in this method the resist must be removed and the catalytic layer underlying the resist must then be etched away to prevent an excessively low surface resistivity due to the catalytic layer on the surface.
In the first of the above-mentioned methods the method is limited to the use of silver salts and a substrate which has preferential adsorption of the silver salt and in the second method, the resist must be removed and an etching step must be employed to remove catalyst underlying the resist. In both methods, however, one may build up the metal deposit while the resist is on the substrate such that the resist provides a channel to prevent widening of the circuit lines to be formed by the depositing metal.
In contrast, U.S. Pat. No. 4,388,351 describes a method for forming a printed circuit board by first providing a negative resist pattern over the surface of the substrate, etching the surface, sensitizing and catalyzing the surface, providing a thin porous flash electroless deposit over the catalyzed substrate, stripping the negative mask together with the catalytic layer and flash coating which overlies the negative mask, thereby leaving a positive flash coated circuit pattern on the substrate and thence electrolessly plating copper to the fullest desired thickness. This method, while advantageous for several reasons and solving some problems in the art has disadvantages of its own. More particularly, one must be extremely careful of the amount of the flash plating deposited in that if it is too thin it does not act in its intended purpose of making a more robust image which allows for greater inventory time before final full build plating. Alternatively, if it is too thick, stripping of the negative mask is difficult. Further, since the negative mask is removed prior to full build electroless plate, the electroless plating not only causes the image to grow in thickness, as desired, but also the image will grow in width as well, thereby preventing or making it extremely difficult to control the desired line width especially when one is dealing with fine line high density printed circuits.
It would therefore be desirable to provide an inexpensive method which is compatible with normal processing such as indicated above. It is also desirable that the photoresist used to define the circuit pattern can remain intact during the full build plating, thereby providing a channel of a defined width preventing spreading as one plated to the desired thickness. Further it is often desirable to eliminate the necessity of stripping the resist and etching away any underlying catalyst.